To ukemi, tap G and a direction (8_6_2_4) when you are knocked to the ground. You don't have to be precise, you can mash G while holding the direction you want to ukemi in.
(Ukemi is also called "teching" locally, referring to the term "tech roll".)
Sometimes you can't ukemi. These are "untechable knockdowns" and I would say every character probably has them. When you are hit with an untechable knockdown you are put in a severely disadvantaged okizeme situation and you must guard or manually sideroll/backroll to avoid attacks, whereas with techable knockdowns you can ukemi out of mixups in most cases.
Examples of untechable knockdowns are Mitsurugi's 2KB BE, or Pyrrha's B+G throw.
